1. Field of the Invention
The present invention relates to a display element which forms a light source, for example, for a single cell of an outdoor large-scaled display unit, and particularly to a color display element provided with a condensing lens with a filter.
2. Description of the Related Art
When a large-screen display unit is formed by arranging monochromatic display tubes each utilizing the light emitted from a fluorescent substance and serving as one pixel in the form of a matrix, it is difficult to improve the resolution because of the occurrence of spaces in the connection portions between the respective display tubes, and an increase in the resolution creates an increase in the production cost. For example, the composite display tube for a light source disclosed in Japanese Patent Laid-Open No. 62-10849 has been thus proposed.
FIGS. 1 to 3 are respectively a front view of the basic structure of such a composite display tube for a light source, a sectional view taken along the line II--II in FIG. 1 and a perspective view of a principal portion thereof. In the drawings, fluorescent screens 5R, 5G, 5B each serve as a single pixel and are arranged in a matrix of 3.times.3 in terms of pixel number. A case 1 comprises a front panel 2 made of a transparent member such as glass, a back panel 3 and a cylindrical side panel 4, the interior being in a vacuum state. Red (R) florescent screens 5R, green (G) fluorescent screens 5G and blue(B) fluorescent screen 5B are arranged in the form of a matrix comprising 3 lines and 3 columns, as shown in FIG. 1, on the back side of the front panel 2. Accelerating electrodes 6 are respectively disposed in the peripheries of the fluorescent screens 5R, 5G and 5B in correspondence with the fluorescent screens. Back electrodes 10 for selecting a line are disposed on the inner side of the back panel 3 in a form with stripes corresponding to the line directions of the fluorescent screens 5R, 5G, 5B. Cathodes 7 are provided above the back electrodes 10 in correspondence with the respective fluorescent screen 5R, 5G, 5B. For example, an indirectly heated cathode in which an oxide is coated on an Ni sleeve, or a directly heated cathode in which an oxide is coated on tungsten is used as each of these cathodes. These cathodes 7 are also supported on the back panel 3 by supporting members 15a, 15b. Control grids 8 for controlling the columns are disposed between the fluorescent screens 5R, 5G, 5B and the cathodes 7 in a form with stripes corresponding to the column directions. Each of the control grids 8 is provided with holes 9 through which electron beams pass. The electron beams 11 are respectively emitted from the cathodes 7 and applied to the fluorescent screens 5R, 5G, 5B. A high voltage is supplied to each of anodes 6 from a terminal 16. A given voltage is supplied to each of the control grids 8 and the back electrodes 10 from terminal pins 12.
In addition, a filter and a condensing lens are provided on the surface of the front panel 2, as shown in FIGS. 4 and 5, so as to prevent any decrease in the contrast owing to the effect of external light (for example, the sunlight or the like) on the fluorescent screens. In FIGS. 4 and 5, on the front panel 2 are provided a filter 13 comprising color filters 13R, 13G, 13B and a condensing lens 14 disposed on the color filters 13R, 13G, 13B, the condensing lens 14 provided with the filter 13 being bonded to the front panel 2 by an adhesive 18a. FIG. 5 shows the case 1 which is mounted on a substrate 17 and which has an interior provided with the cathodes 7, the control grids 8 and the back electrodes 10, which are not shown in the drawing, in the same was as in FIG. 2.
A description will now be given of the operation of the conventional display element. When a negative potential relative to the cathodes 7 is applied to each of the back electrodes 10, since the periphery of each of the cathodes 7 is at a negative potential, the electrons emitted from each of the cathodes 7 do not flow to the control grids 8 and the accelerating anodes 6, thereby bringing the display element into a cut-off state. When a potential of 0 V or a positive potential of several V relative to the cathodes 7 is then applied to the back electrodes 10, the electron beam 11 is emitted from each of the cathodes 7 and flows toward the control grids 8. If the potential of the control grids 8 is negative relative to the cathodes 7, the electron beams 11 respectively cannot pass through the holes 9 and thus cannot reach the accelerating electrodes 6. If the potential of the control grids 8 is positive relative to the cathodes 7, the electron beams respectively pass through the holes 9 and are accelerated by the accelerating electrodes 6 and then applied to the fluorescent screens 5R, 5G, 5B which thus emit light. It is therefore possible to selectively emit light from the fluorescent screens 5R, 5G, 5B, which are disposed at the positions corresponding to the intersections between the back electrodes 10 and the control grids 8 to which voltages are applied by selectively applying a given voltage to the back electrodes 10 for selecting a line in the arrangement of the fluorescent screen 5R, 5G, 5B and the control grids for selecting a column. Further, if a large number of such display elements each serving as one cell are arranged to form a large screen, it is possible to form a color display comprising the fluorescent screens 5R, 5G, 5B each serving as a pixel.
The conventional display element configured as described above has a problem in that, when the filter 13 and the condensing lens 14 are bonded to the surface of the front panel 2 by using the adhesive 18a, the bonding work cannot be easily performed in such a manner that the positions of the color filters 13R, 13G, 13B and the condensing lens 14 respectively agree with the positions of the fluorescent screens 5R, 5G, 5B, and thus easily produces deviations in the positions.